Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Mark Baugher
INTERNET-DRAFT Dan Wing
EXPIRES: December 2005 Cisco Systems
June, 2005
Session Description Protocol Security Descriptionsfor Media Streams<draft-ietf-mmusic-sdescriptions-10.txt>
Status of this memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document defines a Session Description Protocol (SDP)
cryptographic attribute for unicast media streams. The attribute
describes a cryptographic key and other parameters, which serve to
configure security for a unicast media stream in either a single
message or a roundtrip exchange. The attribute can be used with a
variety of SDP media transports and this document defines how to use
it for the Secure Real-time Transport Protocol (SRTP) unicast media
streams. The SDP crypto attribute requires the services of a data
security protocol to secure the SDP message.
Table of Contents
1 Applicability.....................................................32 Notational Conventions............................................33 Introduction......................................................4
INTERNET-DRAFT SDP Security Descriptions June, 2005
4 SDP "Crypto" Attribute and Parameters.............................54.1 Tag............................................................54.2 Crypto-suite...................................................64.3 Key Parameters.................................................64.4 Session Parameters.............................................74.5 Example........................................................75 General Use of the crypto Attribute...............................85.1 Use With Offer/Answer..........................................85.1.1 Generating the Initial Offer - Unicast Streams............85.1.2 Generating the Initial Answer - Unicast Streams...........95.1.3 Processing of the Initial Answer - Unicast Streams.......105.1.4 Modifying the Session....................................105.2 Use Outside Offer/Answer......................................115.3 General Backwards Compatibility Considerations................116 SRTP Security Descriptions.......................................116.1 SRTP Key Parameter............................................126.2 Crypto-suites.................................................156.2.1 AES_CM_128_HMAC_SHA1_80..................................156.2.2 AES_CM_128_HMAC_SHA1_32..................................166.2.3 F8_128_HMAC_SHA1_80......................................166.2.4 Adding new Crypto-suite Definitions......................166.3 Session Parameters............................................166.3.1 KDR=n....................................................166.3.2 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP...................176.3.3 UNAUTHENTICATED_SRTP.....................................176.3.4 FEC_ORDER=order..........................................176.3.5 FEC_KEY=key-params.......................................176.3.6 Window Size Hint (WSH)...................................186.3.7 Defining New SRTP Session Parameters.....................186.4 SRTP Crypto Context Initialization............................186.4.1 Late Binding of one or more SSRCs to a crypto context....206.4.2 Sharing cryptographic contexts among Sessions or SSRCs...216.5 Removal of Crypto Contexts....................................217 SRTP-Specific Use of the crypto Attribute........................217.1 Use with Offer/Answer.........................................217.1.1 Generating the Initial Offer - Unicast Streams...........227.1.2 Generating the Initial Answer - Unicast Streams..........227.1.3 Processing of the Initial Answer - Unicast Streams.......237.1.4 Modifying the Session....................................237.1.5 Offer/Answer Example.....................................247.2 SRTP-Specific Use Outside Offer/Answer........................257.3 Support for SIP Forking.......................................267.4 SRTP-Specific Backwards Compatibility Considerations..........267.5 Operation with KEYMGT= and k= lines...........................278 Security Considerations..........................................278.1 Authentication of packets.....................................288.2 Keystream Reuse...............................................288.3 Signaling Authentication and Signaling Encryption.............289 Grammar..........................................................299.1 Generic "Crypto" Attribute Grammar............................299.2 SRTP "Crypto" Attribute Grammar...............................30Andreasen, Baugher & Wing [Page 2]

INTERNET-DRAFT SDP Security Descriptions June, 200510 IANA Considerations.............................................3110.1 Registration of the "crypto" attribute......................3110.2 New IANA Registries and Registration Procedures.............3110.2.1 Key Method Registry and Registration.....................3110.2.2 Media Stream Transport Registry and Registration.........3210.3 Initial Registrations.......................................3210.3.1 Key Method...............................................3210.3.2 SRTP Media Stream Transport..............................3211 Acknowledgements................................................3312 Authors' Addresses..............................................3313 Normative References............................................3414 Informative References..........................................3415 Full Copyright Statement........................................361 Applicability
RFC YYYY {Note to RFC Editor: replace "YYYY" with RFC number for
[keymgt]} provides similar cryptographic key distribution
capabilities, and is intended for use when the signaling is to be
confidential and/or integrity-protected separately from the keying
material.
In contrast, this specification carries the keying material within
the SDP message, and it is intended for use when the keying material
is protected along with the signaling. Implementations MUST employ
security mechanisms that provide confidentiality and integrity for
the keying material. When this specification is used in the context
of SIP [RFC3261], the application SHOULD employ either the SIPS URI
or S/MIME to provide protection for the SDP message and the keying
material that it contains. The use of transport layer or IP layer
security is NOT RECOMMENDED since the protection of the SDP message
and the keying material that it contains cannot be ensured through
all intermediate entities such as SIP proxies.
2 Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
be interpreted as described in [RFC2119]. The terminology in this
document conforms to [RFC2828], "Internet Security Glossary".
n^r is exponentiation where n is multiplied by itself r times; n and
r are integers. 0..k is an integer range of all integers from 0
through k inclusive.
The terms 'transport' and 'media transport' are used to mean
'transport protocol' as defined in RFC2327, page 20.
Andreasen, Baugher & Wing [Page 3]

INTERNET-DRAFT SDP Security Descriptions June, 2005
Explanatory notes are provided in several places throughout the
document; these notes are indented two spaces from the surrounding
text.
3 Introduction
The Session Description Protocol (SDP) [SDP] describes multimedia
sessions, which can be audio, video, whiteboard, fax, modem, and
other media streams. Security services such as data origin
authentication, integrity and confidentiality are often needed for
those streams. The Secure Real-time Transport Protocol (SRTP) [srtp]
provides security services for RTP media and is signaled by use of
secure RTP transport (e.g., "RTP/SAVP" or "RTP/SAVPF") in an SDP
media (m=) line. However, there are no means within SDP itself to
configure SRTP beyond using default values. This document specifies
a new SDP attribute called "crypto", which is used to signal and
negotiate cryptographic parameters for media streams in general, and
SRTP in particular. The definition of the crypto attribute in this
document is limited to two-party unicast media streams where each
source has a unique cryptographic key; support for multicast media
streams or multipoint unicast streams is for further study.
The crypto attribute is defined in a generic way to enable its use
with SRTP and any other secure transports that can establish
cryptographic parameters with only a single message or in a single
round-trip exchange using the offer/answer model [RFC3264].
Extension to transports other than SRTP, however, is beyond the scope
of this document. Each type of secure media transport needs its own
specification for the crypto-attribute parameter. These definitions
are frequently unique to the particular type of transport and must be
specified in a Standards Track RFC and registered with IANA according
to the procedures defined in Section 10. This document defines the
security parameters and keying material for SRTP only.
It would be self-defeating not to secure cryptographic keys and other
parameters at least as well as the data is secured. Data security
protocols such as SRTP rely upon a separate key management system to
securely establish encryption and/or authentication keys. Key
management protocols provide authenticated key establishment (AKE)
procedures to authenticate the identity of each endpoint and protect
against man-in-the-middle, reflection/replay, connection hijacking
and some denial of service attacks [skeme]. Along with the key, an
AKE protocol such as MIKEY [mikey], GDOI [GDOI], KINK [kink], IKE
[ike], Secure Multiparts [s/mime, pgp/mime] or TLS [TLS] securely
disseminates information describing both the key and the data-
security session. AKE is needed because it is pointless to provide a
key over a medium where an attacker can snoop the key, alter the
definition of the key to render it useless, or change the parameters
of the security session to gain unauthorized access to session-
related information.
Andreasen, Baugher & Wing [Page 4]

INTERNET-DRAFT SDP Security Descriptions June, 2005
SDP, however, was not designed to provide AKE services, and the media
security descriptions defined in this document do not add AKE
services to SDP. This specification is no replacement for a key
management protocol or for the conveyance of key management messages
in SDP [keymgt]. The SDP security descriptions defined here are
suitable for restricted cases only where IPsec, TLS, or some other
encapsulating data-security protocol (e.g., SIP S/MIME) protects the
SDP message. This document adds security descriptions to those
encrypted and/or authenticated SDP messages through the new SDP
"crypto" attribute, which provides the cryptographic parameters of a
media stream.
The "crypto" attribute can be adapted to any media transport, but its
precise definition is unique to a particular transport.
In Section 4, we introduce the general SDP crypto attribute, and in
Section 5 we define how it is used with and without the offer/answer
model. In Section 6, we define the crypto attribute details needed
for SRTP, and in Section 7 we define SRTP-specific use of the
attribute with and without the offer/answer model. Section 8 recites
security considerations, and Section 9 gives an Augmented-BNF grammar
for the general crypto attribute as well as the SRTP-specific use of
the crypto attribute. IANA considerations are provided in Section10.
4 SDP "Crypto" Attribute and Parameters
A new media-level SDP attribute called "crypto" describes the
cryptographic suite, key parameters, and session parameters for the
preceding unicast media line. The "crypto" attribute MUST only
appear at the SDP media level (not at the session level). The
"crypto" attribute follows the format (see Section 9.1 for the formal
ABNF grammar):
a=crypto:<tag> <crypto-suite> <key-params> [<session-params>]
The fields tag, crypto-suite, key-params, and session-params are
described in the following sub-sections. Below we show an example of
the crypto attribute for the "RTP/SAVP" transport, i.e., the secure
RTP extension to the Audio/Video Profile [srtp]. In the following,
newlines are included for formatting reasons only:
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32
The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined
by the text starting with "inline:", and session-params is omitted.
4.1 TagAndreasen, Baugher & Wing [Page 5]

INTERNET-DRAFT SDP Security Descriptions June, 2005
The tag is a decimal number used as an identifier for a particular
crypto attribute (see Section 9.1 for details). The tag MUST be
unique among all crypto attributes for a given media line. It is
used with the offer/answer model to determine which of several
offered crypto attributes were chosen by the answerer (see Section5.1).
In the offer/answer model, the tag is a negotiated parameter.
4.2 Crypto-suite
The crypto-suite field is an identifier that describes the encryption
and authentication algorithms (e.g., AES_CM_128_HMAC_SHA1_80) for
the transport in question (see Section 9.1 for details). The
possible values for the crypto-suite parameter are defined within the
context of the transport, i.e., each transport defines a separate
namespace for the set of crypto-suites. For example, the crypto-
suite "AES_CM_128_HMAC_SHA1_80" defined within the context
"RTP/SAVP" transport applies to Secure RTP only; the string may be
reused for another transport (e.g., "RTP/SAVPF" [srtpf]), but a
separate definition would be needed.
In the offer/answer model, the crypto-suite is a negotiated
parameter.
4.3 Key Parameters
The key-params field provides one or more sets of keying material for
the crypto-suite in question. The field consists of a method
indicator followed by a colon, and the actual keying information as
shown below (the formal grammar is provided in Section 9.1):
key-params = <key-method> ":" <key-info>
Keying material might be provided by different means than key-params,
however this is out of scope. Only one method is defined in this
document, namely "inline", which indicates that the actual keying
material is provided in the key-info field itself. There is a single
name space for the key-method, i.e., the key-method is transport
independent. New key-methods (e.g., use of a URL) may be defined in
a Standards Track RFC in the future. Although the key-method itself
may be generic, the accompanying key-info definition is specific not
only to the key-method, but also to the transport in question. Key-
info encodes keying material for a crypto suite, which defines that
keying material. New key methods MUST be registered with the IANA
according to the procedures defined in Section 10.2.1.
Key-info is defined as a general character string (see Section 9.1
for details); further transport and key-method specific syntax and
semantics MUST be provided in a Standards Track RFC for each
combination of transport and key-method that use it; definitions for
Andreasen, Baugher & Wing [Page 6]

INTERNET-DRAFT SDP Security Descriptions June, 2005
SRTP are provided in Section 6. Note that such definitions are
provided within the context of both a particular transport (e.g.,
"RTP/SAVP") and a specific key-method (e.g., "inline"). IANA will
register the list of supported key methods for each transport.
When multiple keys are included in the key parameters, it MUST be
possible to determine which of the keys is being used in a given
media packet by a simple inspection of the media packet received; a
trial-and-error approach between the possible keys MUST NOT be
performed.
For SRTP, this could be achieved by use of Master Key Identifiers
(MKI) [srtp]. Use of <"From, "To"> values are not supported in
SRTP security descriptions for reasons explained in Section 6.1,
below.
In the offer/answer model, the key parameter is a declarative
parameter.
4.4 Session Parameters
Session parameters are specific to a given transport and use of them
is OPTIONAL in the security descriptions framework, where they are
just defined as general character strings. If session parameters are
to be used for a given transport, then transport-specific syntax and
semantics MUST be provided in a Standards Track RFC; definitions for
SRTP are provided in Section 6.
In the offer/answer model, session parameters may be either
negotiated or declarative; the definition of specific session
parameters MUST indicate whether they are negotiated or declarative.
Negotiated parameters apply to data sent in both directions, whereas
declarative parameters apply only to media sent by the entity that
generated the SDP. Thus, a declarative parameter in an offer applies
to media sent by the offerer, whereas a declarative parameter in an
answer applies to media sent by the answerer.
4.5 Example
This example shows use of the crypto attribute for the "RTP/SAVP"
media transport type (as defined in Section 5). The "a=crypto" line
is actually one long line; it is shown as two lines due to page
formatting:
Andreasen, Baugher & Wing [Page 7]

INTERNET-DRAFT SDP Security Descriptions June, 2005
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 161.44.17.12/127
t=2873397496 2873404696
m=video 51372 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
m=application 32416 udp wb
a=orient:portrait
This SDP message describes three media streams, two of which use the
"RTP/SAVP" transport. Each has a crypto attribute for the
"RTP/SAVP" transport. These secure-RTP specific descriptions are
defined in Section 6.
5 General Use of the crypto Attribute
In this section, we describe the general use of the crypto attribute
outside of any transport or key-method specific rules.
5.1 Use With Offer/Answer
The general offer/answer rules for the crypto attribute are in
addition to the rules specified in RFC 3264, which MUST be followed,
unless otherwise noted. RFC 3264 defines operation for both unicast
and multicast streams; the sections below describe operation for two-
party unicast streams only, since support for multicast streams (and
multipoint unicast streams) is for further study.
5.1.1 Generating the Initial Offer - Unicast Streams
When generating an initial offer for a unicast stream, there MUST be
one or more crypto attributes present for each media stream for which
security is desired. Each crypto attribute for a given media stream
MUST contain a unique tag.
The ordering of multiple "a=crypto" lines is significant: The most
preferred crypto line is listed first. Each crypto attribute
describes the crypto-suite, key(s) and possibly session parameters
offered for the media stream. In general, a "more preferred" crypto-
suite SHOULD be cryptographically stronger than a "less preferred"
crypto-suite.
Andreasen, Baugher & Wing [Page 8]

INTERNET-DRAFT SDP Security Descriptions June, 2005
The crypto-suite always applies to media in the directions supported
by the media stream (e.g., send and receive). The key(s), however,
apply to media in the direction from the offerer to the answerer; if
the media stream is marked as "recvonly", a key MUST still be
provided.
This is done for consistency. Also, in the case of SRTP, for
example, secure RTCP will still be flowing in both the send and
receive direction for a unidirectional stream.
The offer may include session parameters. There are no general offer
rules for the session parameters; instead, specific rules may be
provided as part of the transport-specific definitions of any session
parameters.
When issuing an offer, the offerer MUST be prepared to support media
security in accordance with any of the crypto attributes included in
the offer. There are however two problems associated with this.
First of all, the offerer does not know which key the answerer will
be using for media sent to the offerer. Second, the offerer may not
be able to deduce which of the offered crypto attributes were
accepted. Since media may arrive prior to the answer, delay or
clipping can occur. If this is unacceptable to the offerer, the
offerer SHOULD use a mechanism outside the scope of this document to
prevent the above problem.
For example, in SIP [RFC3261], a "security" precondition as defined
in [sprecon] could solve the above problem.
5.1.2 Generating the Initial Answer - Unicast Streams
When the answerer receives the initial offer with one or more crypto
attributes for a given unicast media stream, the answerer MUST either
accept exactly one of the offered crypto attributes, or the offered
stream MUST be rejected.
If the answerer wishes to indicate support for other crypto
attributes, those can be listed by use of the SDP Simple Capability
Declaration [RFC3407] extensions.
Only crypto attributes that are valid can be accepted; valid
attributes do not violate any of the general rules defined for
security descriptions as well as any specific rules defined for the
transport and key-method in question. When selecting one of the
valid crypto attributes, the answerer SHOULD select the most
preferred crypto attribute it can support, i.e., the first valid
supported crypto attribute in the list, considering the answerer's
capabilities and security policies.
Andreasen, Baugher & Wing [Page 9]

INTERNET-DRAFT SDP Security Descriptions June, 2005
If there are one or more crypto attributes in the offer, but none of
them are valid, or none of the valid ones are supported, the offered
media stream MUST be rejected.
When an offered crypto attribute is accepted, the crypto attribute in
the answer MUST contain the following:
* The tag and crypto-suite from the accepted crypto attribute in the
offer (the same crypto-suite MUST be used in the send and receive
direction).
* The key(s) the answerer will be using for media sent to the
offerer. Note that a key MUST be provided, irrespective of any
direction attributes in the offer or answer.
Furthermore, any session parameters that are negotiated MUST be
included in the answer. Declarative session parameters provided by
the offerer are not included in the answer, however the answerer may
provide its own set of declarative session parameters.
Once the answerer has accepted one of the offered crypto attributes,
the answerer MAY begin sending media to the offerer in accordance
with the selected crypto attribute. Note however, that the offerer
may not be able to process such media packets correctly until the
answer has been received.
5.1.3 Processing of the Initial Answer - Unicast Streams
When the offerer receives the answer, the offerer MUST verify, that
one of the initially offered crypto suites and its accompanying tag
was accepted and echoed in the answer. Also, the answer MUST include
one or more keys, which will be used for media sent from the answerer
to the offerer.
If the offer contained any mandatory negotiated session parameters
(see section 6.3.7), the offerer MUST verify that said parameters are
included in the answer and support them. If the answer contains any
mandatory declarative session parameters, the offerer MUST be able to
support those.
If any of the above fails, the negotiation MUST fail.
5.1.4 Modifying the Session
Once a media stream has been established, it MAY be modified at any
time, as described in RFC 3264, Section 8. Such a modification MAY
be triggered by the security service, e.g., in order to perform a re-
keying or change the crypto-suite. If media stream security using
the general security descriptions defined here is still desired, the
crypto attribute MUST be included in these new offer/answer
exchanges. The procedures are similar to those defined in Section
Andreasen, Baugher & Wing [Page 10]

INTERNET-DRAFT SDP Security Descriptions June, 2005
5.1.1, 5.1.2, 5.1.3 of this document, subject to the considerations
provided in RFC 3264 Section 8.
5.2 Use Outside Offer/Answer
The crypto attribute can also be used outside the context of
offer/answer where there is no negotiation of the crypto suite,
cryptographic key or session parameters. In this case, the sender
determines security parameters for the stream. Since there is no
negotiation mechanisms, the sender MUST include exactly one crypto
attribute and the receiver MUST either accept it or else SHOULD NOT
receive the associated stream. The sender SHOULD select the security
description that it deems most secure for its purposes.
5.3 General Backwards Compatibility Considerations
In the offer/answer model, it is possible that the answerer supports
a given secure transport (e.g., "RTP/SAVP") and accepts the offered
media stream, yet the answerer does not support the crypto attribute
defined in this document and hence ignores it. The offerer can
recognize this situation by seeing an accepted media stream in the
answer that does not include a crypto line. In that case, the
security negotiation defined here MUST fail.
Similar issues exist when security descriptions are used outside of
the offer/answer model. But the source of a non-negotiated security
description has no indication that the receiver has ignored the
crypto attribute.
6 SRTP Security Descriptions
In this section, we provide definitions for security descriptions for
SRTP media streams. In the next section, we define how to use SRTP
security descriptions with and without the offer/answer model.
SRTP security descriptions MUST only be used with the SRTP transport
(e.g., "RTP/SAVP" or "RTP/SAVPF"). The following specifies security
descriptions for the "RTP/SAVP" profile defined in [srtp], however it
is expected that other secure RTP profiles (e.g., "RTP/SAVPF") can
use the same descriptions, which are in accordance with the SRTP
protocol specification [srtp].
There is no assurance that an endpoint is capable of configuring its
SRTP service with a particular crypto attribute parameter, but SRTP
guarantees minimal interoperability among SRTP endpoints through the
default SRTP parameters [srtp]. More capable SRTP endpoints support
a variety of parameter values beyond the SRTP defaults and these
values can be configured by the SRTP security descriptions defined
here. An endpoint that does not support the crypto attribute will
ignore it according to the SDP. Such an endpoint will not correctly
process the particular media stream. By using the Offer/Answer
Andreasen, Baugher & Wing [Page 11]

INTERNET-DRAFT SDP Security Descriptions June, 2005
model, the offerer and answerer can negotiate the crypto parameters
to be used before commencement of the multimedia session (see Section7.1).
There are over twenty cryptographic parameters listed in the SRTP
specification. Many of these parameters have fixed values for
particular cryptographic transforms. At the time of session
establishment, however, there is usually no need to provide unique
settings for many of the SRTP parameters, such as salt length and
pseudo-random function (PRF). Thus, it is possible to simplify the
list of parameters by defining "cryptographic suites" that fix a set
of SRTP parameter values for the security session. This approach is
followed by the SRTP security descriptions, which uses the general
security description parameters as follows:
* crypto-suite: Identifies the encryption and authentication
transforms
* key parameter: SRTP keying material and parameters
* session parameters: The following parameters are defined:
- KDR: The SRTP Key Derivation Rate is the rate that a
pseudo-random function is applied to a master key
- UNENCRYPTED_SRTP: SRTP messages are not encrypted
- UNENCRYPTED_SRTCP: SRTCP messages are not encrypted
- UNAUTHENTICATED_SRTP: SRTP messages are not authenticated
- FEC_ORDER: Order of forward error correction (FEC)
relative to SRTP services
- FEC_KEY: Master Key for FEC when the FEC stream is sent
to a separate address and/or port.
- WSH: Window Size Hint
- Extensions: Extension parameters can be defined
Please refer to the SRTP specification for a complete list of
parameters and their descriptions [Section 8.2, srtp]. Regarding the
UNENCRYPTED_SRTCP parameter, offerers and answerers of SDP security
descriptions MUST NOT use the SRTCP E-bit to override
UNENCRYPTED_SRTCP or the default, which is to encrypt all SRTCP
messages (see Section 6.3.2). The key parameter, the crypto-suite,
and the session parameters shown above are described in detail in the
following subsections.
6.1 SRTP Key Parameter
SRTP security descriptions define use of the "inline" key method as
described in the following. Use of any other keying method, e.g.,
URL, for SRTP security descriptions is for further study.
The "inline" type of key contains the keying material (master key
and salt) and all policy related to that master key, including how
long it can be used (lifetime) and whether or not it uses a master
key identifier (MKI) to associate an incoming SRTP packet with a
particular master key. Compliant implementations obey the policies
Andreasen, Baugher & Wing [Page 12]

INTERNET-DRAFT SDP Security Descriptions June, 2005
associated with a master key, and MUST NOT accept incoming packets
that violate the policy (e.g., after the master key lifetime has
expired).
The key parameter contains one or more cryptographic master keys,
each of which MUST be a unique cryptographically random [RFC1750]
value with respect to other master keys in the entire SDP message
(i.e., including master keys for other streams). Each key follows
the format (the formal definition is provided in Section 9.2):
"inline:" <key||salt> ["|" lifetime] ["|" MKI ":" length]
key||salt concatenated master key and salt, base64 encoded
(see [RFC3548], Section 3)
lifetime master key lifetime (max number of SRTP or SRTCP
packets using this master key)
MKI:length MKI and length of the MKI field in SRTP packets
The following definition provides an example for
AES_CM_128_HMAC_SHA1_80:
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:4
The first field ("d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj") of the
parameter is the cryptographic master key appended with the master
salt; the two are first concatenated and then base64 encoded. The
length of the concatenated key and salt is determined by the crypto-
suite for which the key applies. If the length (after being decoded
from base64) does not match that specified for the crypto-suite, the
crypto attribute in question MUST be considered invalid. Each master
key and salt MUST be a cryptographically random number and MUST be
unique to the entire SDP message. When base64 decoding the key and
salt, padding characters (i.e., one or two "=" at the end of the
base64 encoded data) are discarded (see [RFC3548] for details).
Base64 encoding assumes that the base64 encoding input is an integral
number of octets. If a given crypto-suite requires the use of a
concatenated key and salt with a length that is not an integral
number of octets, said crypto-suite MUST define a padding scheme that
results in the base64 input being an integral number of octets. For
example, if the length defined was 250 bits, then 6 padding bits
would be needed, which could be defined to be the last 6 bits in a
256 bit input.
The second field, is the OPTIONAL lifetime of the master key as
measured in maximum number of SRTP or SRTCP packets using that master
key (i.e., the number of SRTP packets and the number of SRTCP packets
each have to be less than the lifetime). The lifetime value MAY be
written as a non-zero, positive integer or as a power of 2 (see the
grammar in Section 9.2 for details). The "lifetime" value MUST NOT
exceed the maximum packet lifetime for the crypto-suite. If the
lifetime is too large or otherwise invalid then the entire crypto
Andreasen, Baugher & Wing [Page 13]

INTERNET-DRAFT SDP Security Descriptions June, 2005
attribute MUST be considered invalid. The default MAY be implicitly
signaled by omitting the lifetime (note that the lifetime field never
includes a colon, whereas the third field always does). This is
convenient when the SRTP cryptographic key lifetime is the default
value. As a shortcut to avoid long decimal values, the syntax of the
lifetime allows using the literal "2^", which indicates "two to the
power of". The example above, shows a case where the lifetime is
specified as 2^20. The following example, which is for the
AES_CM_128_HMAC_SHA1_80 crypto-suite, has a default for the lifetime
field, which means that SRTP's and SRTCP's default values will be
used (see [srtp]):
inline:YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6MTIzNDU2|1066:4
The example shows a 30-character key and concatenated salt that is
base64 encoded: The 30-character key/salt concatenation is expanded
to 40 characters by the three-in-four encoding of base64.
The third field, which is also OPTIONAL, is the Master Key Identifier
(MKI) and its byte length.
"MKI" is the master key identifier associated with the SRTP master
key. The MKI is here defined as a positive integer which is encoded
as a big-endian integer in the actual SRTP packets. If the MKI is
given, then the length of the MKI MUST also be given and separated
from the MKI by a colon (":"). The MKI length is the size of the MKI
field in the SRTP packet, specified in bytes. If the MKI length is
not given or its value exceeds 128 (bytes), then the entire crypto
attribute MUST be considered invalid. The substring "1:4" in the
first example assigns to the key a master key identifier of 1 that is
4 bytes long, and the second example assigns a 4-byte master key
identifier of 1066 to the key. One or more master keys with their
associated MKI can be initially defined, and then later updated, or
deleted and new ones defined.
SRTP offers a second feature for specifying the lifetime of a master
key in terms of two values, called "From" and "To," which are defined
on the SRTP sequence number space [srtp]. This SRTP Security
Descriptions specification, however, does not support the
<"From","To"> feature since the lifetime of an AES master key is 2^48
SRTP packets, which means that there is no cryptographic reason to
replace a master key for practical point-to-point applications. For
this reason, there is no need to support two means for signaling key
update. The MKI is chosen over <"From","To"> by this specification
for the very few applications that need it since the MKI feature is
simpler (though the MKI adds additional bytes to each packet whereas
<"From", "To"> does not).
As mentioned above, the key parameter can contain one or more master
keys. When the key parameter contains more than one master key, all
of the master keys in that key parameter MUST include an MKI value.
Andreasen, Baugher & Wing [Page 14]

INTERNET-DRAFT SDP Security Descriptions June, 2005
The length of the base64 decoded key and salt value for this crypto-
suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto
attribute is considered invalid.
6.2.2 AES_CM_128_HMAC_SHA1_32
This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except
that the authentication tag is 32 bits.
The length of the base64-decoded key and salt value for this crypto-
suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto
attribute is considered invalid.
6.2.3 F8_128_HMAC_SHA1_80
This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except the
cipher is F8 [srtp].
The length of the base64 decoded key and salt value for this crypto-
suite MUST be 30 characters, i.e. 240 bits; otherwise the crypto
attribute is considered invalid.
6.2.4 Adding new Crypto-suite Definitions
If new transforms are added to SRTP, new definitions for those
transforms SHOULD be given for the SRTP security descriptions and
published in a Standards Track RFC. Sections 6.2.1 through 6.2.3
illustrate how to define crypto-suite values for particular
cryptographic transforms. Any new crypto-suites MUST be registered
with IANA following the procedures in section 10.
6.3 Session Parameters
SRTP security descriptions define a set of "session" parameters,
which OPTIONALLY may be used to override SRTP session defaults for
the SRTP and SRTCP streams. These parameters configure an RTP
session for SRTP services. The session parameters provide session-
specific information to establish the SRTP cryptographic context.
6.3.1 KDR=n
KDR specifies the Key Derivation Rate, as described in section 4.3.1
of [srtp].
The value n MUST be an integer in the set {1,2,...,24}, which
denotes a power of 2 from 2^1 to 2^24, inclusive. The SRTP key
derivation rate controls how frequently a new session key is derived
from an SRTP master key(s) [srtp] given in the declaration. When
the key derivation rate is not specified (i.e., the KDR parameter is
omitted), a single initial key derivation is performed [srtp].
Andreasen, Baugher & Wing [Page 16]

INTERNET-DRAFT SDP Security Descriptions June, 2005
In the offer/answer model, KDR is a declarative parameter.
6.3.2 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP
SRTP and SRTCP packet payloads are encrypted by default. The
UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP session parameters modify the
default behavior of the crypto-suites with which they are used:
* UNENCRYPTED_SRTCP signals that the SRTCP packet payloads are not
encrypted.
* UNENCRYPTED_SRTP signals that the SRTP packet payloads are not
encrypted.
In the offer/answer model, these parameters are negotiated. If
UNENCRYPTED_SRTCP is signaled for the session, then the SRTCP E bit
MUST be clear (0) in all SRTCP messages. If the default is used,
all SRTCP messages are encrypted and the E bit MUST be set (1) on
all SRTCP messages.
6.3.3 UNAUTHENTICATED_SRTP
SRTP and SRTCP packet payloads are authenticated by default. The
UNAUTHENTICATED_SRTP session parameter signals that SRTP messages
are not authenticated. Use of UNAUTHENTICATED_SRTP is NOT
RECOMMENDED (see Security Considerations).
The SRTP specification requires use of message authentication for
SRTCP, but not for SRTP [srtp].
In the offer/answer model, this parameter is negotiated.
6.3.4 FEC_ORDER=order
FEC_ORDER signals the use of forward error correction for the RTP
packets [rfc2733]. The forward error correction values for "order"
are FEC_SRTP or SRTP_FEC. FEC_SRTP signals that FEC is applied
before SRTP processing by the sender of the SRTP media and after SRTP
processing by the receiver of the SRTP media; FEC_SRTP is the
default. SRTP_FEC is the reverse processing.
In the offer/answer model, FEC_ORDER is a declarative parameter.
6.3.5 FEC_KEY=key-params
FEC_KEY signals the use of separate master key(s) for a Forward
Error Correction (FEC) stream. The master key(s) are specified with
the exact same format as the SRTP Key Parameter defined in Section6.1, and the semantic rules are the same - in particular, the master
key(s) MUST be different from all other master key(s) in the SDP. A
FEC_KEY MUST be specified when the FEC stream is sent to a different
Andreasen, Baugher & Wing [Page 17]

INTERNET-DRAFT SDP Security Descriptions June, 2005
IP-address and/or port than the media stream to which it applies
(i.e., the "m=" line), e.g., as described in RFC 2733 Section 11.1.
When a FEC stream is sent to the same IP-address and port as the
media stream to which it applies, a FEC_KEY MUST NOT be specified.
If a FEC_KEY is specified in this latter case, the crypto attribute
in question MUST be considered invalid.
In the offer/answer model, FEC_KEY is a declarative parameter.
6.3.6 Window Size Hint (WSH)
SRTP defines the SRTP-WINDOW-SIZE [srtp, section 3.3.2] parameter to
protect against replay attacks. The minimum value is 64 [srtp],
however this value may be considered too low for some applications
(e.g., video).
The Window Size Hint (WSH) session parameter provides a hint for how
big this window should be to work satisfactorily (e.g., based on
sender knowledge of number of packets per second). However, there
might be enough information given in SDP attributes like
"a=maxprate" [maxprate] and the bandwidth modifiers to allow a
receiver to derive the parameter satisfactorily. Consequently, this
value is only considered a hint to the receiver of the SDP which MAY
choose to ignore the value provided.
In the offer/answer model, WSH is a declarative parameter.
6.3.7 Defining New SRTP Session Parameters
New SRTP session parameters for the SRTP security descriptions can
be defined in a Standards Track RFC and registered with IANA
according to the registration procedures defined in Section 10.
New SRTP session parameters are by default mandatory. A newly-
defined SRTP session parameter that is prefixed with the dash
character ("-") however is considered optional and MAY be ignored.
If an SDP crypto attribute is received with an unknown session
parameter that is not prefixed with a "-" character, that crypto
attribute MUST be considered invalid.
6.4 SRTP Crypto Context Initialization
In addition to the various SRTP parameters defined above, there are
three pieces of information that are critical to the operation of the
default SRTP ciphers:
* SSRC: Synchronization source
* ROC: Roll-over counter for a given SSRC
* SEQ: Sequence number for a given SSRC
Andreasen, Baugher & Wing [Page 18]

INTERNET-DRAFT SDP Security Descriptions June, 2005
In a unicast session, as defined here, there are three constraints on
these values.
The first constraint is on the SSRC, which makes an SRTP keystream be
unique from other participants. As explained in SRTP, the keystream
MUST NOT be reused on two or more different pieces of plaintext.
Keystream reuse makes the ciphertext vulnerable to cryptanalysis.
One vulnerability is that known-plaintext fields in one stream can
expose portions of the reused keystream and this could further expose
more plaintext in other streams. Since all current SRTP encryption
transforms use keystreams, key sharing is a general problem [srtp].
SRTP mitigates this problem by including the SSRC of the sender in
the keystream. But SRTP does not solve this problem in its entirety
because Real-time Transport Protocol has SSRC collisions, which are
very rare [rtp] but quite possible. During a collision, two or more
SSRCs that share a master key will have identical keystreams for
overlapping portions of the RTP sequence-number space. SRTP Security
Descriptions avoids keystream reuse by making unique master keys
REQUIRED for the sender and receiver of the security description.
Thus, the first constraint is satisfied.
Also note, that there is a second problem with SSRC collisions: The
SSRC is used to identify the crypto context and thereby the cipher,
key, ROC, etc. to process incoming packets. In case of SSRC
collisions, crypto context identification becomes ambiguous and
correct packet processing may not occur. Furthermore, if an RTCP
BYE packet is to be sent for a colliding SSRC, that packet may also
have to be secured. In a (unicast) point-to-multipoint scenario,
this can be problematic for the same reasons, i.e., it is not known
which of the possible crypto contexts to use. Note that these
problems are not unique to the SDP security descriptions; any use
of SRTP needs to consider them.
The second constraint is that the ROC MUST be zero at the time that
each SSRC commences sending packets. Thus, there is no concept of a
"late joiner" in SRTP security descriptions, which are constrained to
be unicast and pairwise. The ROC and SEQ form a "packet index" in
the default SRTP transforms and the ROC is consistently set to zero
at session commencement, according to this document.
The third constraint is that the initial value of SEQ SHOULD be
chosen to be within the range of 0..2^15-1; this avoids an ambiguity
when packets are lost at the start of the session. If at the start
of a session, an SSRC source might randomly select a high sequence-
number value and put the receiver in an ambiguous situation: If
initial packets are lost in transit up to the point that the sequence
number wraps (i.e., exceeds 2^16-1), then the receiver might not
recognize that its ROC needs to be incremented. By restricting the
initial SEQ to the range of 0..2^15-1, SRTP packet-index
determination will find the correct ROC value, unless all of the
first 2^15 packets are lost (which seems, if not impossible, then
Andreasen, Baugher & Wing [Page 19]

INTERNET-DRAFT SDP Security Descriptions June, 2005
rather unlikely). See Section 3.3.1 of the SRTP specification
regarding packet-index determination [srtp].
6.4.1 Late Binding of one or more SSRCs to a crypto context
The packet index, therefore, depends on the SSRC, the SEQ of an
incoming packet and the ROC, which is an SRTP crypto context
variable. Thus, SRTP has a big security dependency on SSRC
uniqueness.
Given the above constraints, unicast SRTP crypto contexts can be
established without the need to negotiate SSRC values in the SRTP
security descriptions. Instead, an approach called "late binding" is
RECOMMENDED by this specification. When a packet arrives, the SSRC
that is contained in it can be bound to the crypto context at the
time of session commencement (i.e., SRTP packet arrival) rather than
at the time of session signaling (i.e., receipt of an SDP). With the
arrival of the packet containing the SSRC, all the data items needed
for the SRTP crypto context are held by the receiver (note that the
ROC value by definition is zero; if non-zero values were to be
supported, additional signaling would be required). In other words,
the crypto context for a secure RTP session using late binding is
initially identified by the SDP as:
<*, address, port>
where '*' is a wildcard SSRC, "address" is the local receive address
from the "c=" line, and "port" is the local receive port from the
"m=" line. When the first packet arrives with ssrcX in its SSRC
field, the crypto context
<ssrcX, address, port>
is instantiated subject to the following constraints:
* Media packets are authenticated: Authentication MUST succeed;
otherwise, the crypto context is not instantiated.
* Media packets are not authenticated: Crypto context is
automatically instantiated.
It should be noted, that use of late binding when there is no
authentication of the SRTP media packets is subject to numerous
security attacks and consequently it is NOT RECOMMENDED (of course,
this can be said for unauthenticated SRTP in general).
Note that use of late binding without authentication will result in
local state being created as a result of receiving a packet from
any unknown SSRC. UNAUTHENTICATED_SRTP, therefore is NOT
RECOMMENDED because it invites easy denial-of-service attack. In
Andreasen, Baugher & Wing [Page 20]

INTERNET-DRAFT SDP Security Descriptions June, 2005
contrast, late binding with authentication does not suffer from
this weakness.
6.4.2 Sharing cryptographic contexts among Sessions or SSRCs
With the constraints and procedures described above, it is not
necessary to explicitly signal the SSRC, ROC and SEQ for a unicast
RTP session. So there are no a=crypto parameters for signaling SSRC,
ROC or SEQ. Thus, multiple SSRCs from the same entity will share
a=crypto parameters when late binding is used. Multiple SSRCs from
the same entity arises due to either multiple sources (microphones,
cameras, etc.), or RTP payloads requiring SSRC multiplexing within
that same session. SDP also allows multiple RTP sessions to be
defined in the same media description ("m="), these RTP sessions will
also share the a=crypto parameters. An application that uses
a=crypto in this way serially shares a master key among RTP sessions
or SSRCs and MUST replace the master key when the aggregate number of
packets among all SSRCs approaches 2^31 packets. SSRCs that share a
master key MUST be unique from one another.
6.5 Removal of Crypto Contexts
The mechanism defined above addresses the issue of creating crypto
contexts, however in practice, session participants may want to
remove crypto contexts prior to session termination. Since a crypto
context contains information that can not automatically be recovered
(e.g., ROC), it is important that the sender and receiver agree on
when a crypto context can be removed, and perhaps more importantly
when it cannot.
Even when late binding is used for a unicast stream, the ROC is
lost and cannot be recovered automatically (unless it is zero) once
the crypto context is removed.
We resolve this problem as follows. When SRTP security descriptions
are being used, crypto-context removal MUST follow the same rules as
SSRC removal from the member table [RFC 3550]; note that this can
happen as the result of an SRTCP BYE packet or a simple time-out due
to inactivity. Inactive session participants that wish to ensure
their crypto contexts are not timed out MUST thus send SRTCP packets
at regular intervals.
7 SRTP-Specific Use of the crypto AttributeSection 5 describes general use of the crypto attribute, and this
section completes it by describing SRTP-specific use.
7.1 Use with Offer/Answer
In this section, we describe how the SRTP security descriptions are
used with the offer/answer model to negotiate cryptographic
Andreasen, Baugher & Wing [Page 21]

INTERNET-DRAFT SDP Security Descriptions June, 2005
capabilities and communicate SRTP master keys. The rules defined
below complement the general offer/answer rules defined in Section5.1, which MUST be followed, unless otherwise specified. Note that
the rules below define unicast operation only; support for multicast
and multipoint unicast streams is for further study.
7.1.1 Generating the Initial Offer - Unicast Streams
When the initial offer is generated, the offerer MUST follow the
steps in Section 5.1.1 as well as the following steps.
For each unicast media line (m=) using the secure RTP transport
where the offerer wants to specify cryptographic parameters, the
offerer MUST provide at least one valid SRTP security description
("a=crypto" line), as defined in Section 6. If the media stream
includes Forward Error Correction with a different IP-address and/or
port than the media stream itself, a FEC_KEY parameter MUST be
included, as described in Section 6.3.5.
The offerer MAY include one or more other SRTP session parameters as
defined in Section 6.3. Note however, that if any SRTP session
parameters are included that are not known to the answerer, but are
nonetheless mandatory (see Section 6.3.6), the negotiation will fail
if the answerer does not support them.
7.1.2 Generating the Initial Answer - Unicast Streams
When the initial answer is generated, the answerer MUST follow the
steps in Section 5.1.2 as well as the following steps.
For each unicast media line which uses the secure RTP transport and
contains one or more "a=crypto" lines in the offer, the answerer
MUST either accept one (and only one) of the crypto lines for that
media stream, or it MUST reject the media stream. Only "a=crypto"
lines that are considered valid SRTP security descriptions as
defined in Section 6 can be accepted. Furthermore, all parameters
(crypto-suite, key parameter, and mandatory session parameters) MUST
be acceptable to the answerer in order for the offered media stream
to be accepted. Note that if the media stream includes Forward
Error Correction with a different IP-address and/or port than the
media stream itself, a FEC_KEY parameter MUST be included, as
described in Section 6.3.5.
When the answerer accepts an SRTP unicast media stream with a crypto
line, the answerer MUST include one or more master keys appropriate
for the selected crypto algorithm; the master key(s) included in the
answer MUST be different from those in the offer.
When the master key(s) are not shared between the offerer and
answerer, SSRC collisions between the offerer and answerer will not
Andreasen, Baugher & Wing [Page 22]

INTERNET-DRAFT SDP Security Descriptions June, 2005
lead to keystream reuse, and hence SSRC collisions do not
necessarily have to be prevented.
If Forward Error Correction to a separate IP-address and/or port is
included, the answer MUST include a FEC_KEY parameter, as described
in Section 6.3.5.
Declarative session parameters may be added to the answer as usual,
however the answerer SHOULD NOT add any mandatory session parameter
(see Section 6.3.6) that might be unknown to the offerer.
If the answerer cannot find any valid crypto line that it supports,
or if its configured policy prohibits any cryptographic key
parameter (e.g., key length) or cryptographic session parameter
(e.g., KDR, FEC_ORDER), it MUST reject the media stream, unless it
is able to successfully negotiate use of SRTP by other means outside
the scope of this document (e.g., by use of MIKEY [mikey]).
7.1.3 Processing of the Initial Answer - Unicast Streams
When the offerer receives the answer, it MUST perform the steps in
Section 5.1.3 as well as the following steps for each SRTP media
stream it offered with one or more crypto lines in it.
If the media stream was accepted and it contains a crypto line, it
MUST be checked that the crypto line is valid according to the
constraints specified in Section 6 (including any FEC constraints).
If the offerer either does not support or is not willing to honor
one or more of the SRTP parameters in the answer, the offerer MUST
consider the crypto line invalid.
If the crypto line is not valid, or the offerer's configured policy
prohibits any cryptographic key parameter (e.g. key length) or
cryptographic session parameter, the SRTP security negotiation MUST
be deemed to have failed.
7.1.4 Modifying the Session
When a media stream using the SRTP security descriptions has been
established, and a new offer/answer exchange is performed, the
offerer and answerer MUST follow the steps in Section 5.1.4 as well
as the following steps.
When modifying the session, all negotiated aspects of the SRTP media
stream can be modified. For example, a new crypto suite can be used
or a new master key can be established. As described in RFC 3264,
when doing a new offer/answer exchange there will be a window of
time, where the offerer and the answerer must be prepared to receive
media according to both the old and the new offer/answer exchange.
Andreasen, Baugher & Wing [Page 23]

INTERNET-DRAFT SDP Security Descriptions June, 2005
This requirement applies here as well, however the following should
be noted:
* When authentication is not being used, it may not be possible for
either the offerer or the answerer to determine if a given packet
is encrypted according to the old or new offer/answer exchange.
RFC 3264 defines a couple of techniques to address this problem,
e.g., changing the payload types used and/or the transport
addresses. Note however that a change in transport addresses may
have an impact on Quality of Service as well as firewall and NAT
traversal. The SRTP security descriptions use the MKI to deal with
this (which adds a few bytes to each SRTP packet) as described in
Section 6.1. For further details on the MKI, please refer to
[srtp].
* If the answerer changes its master key, the offerer will not
be able to process packets secured via this master key until the
answer is received. This could be addressed by using a security
"precondition" [sprecon].
If the offerer includes an IP address and/or port that differs from
that used previously for a media stream (or FEC stream), the offerer
MUST include a new master key with the offer (and in so doing, it
will be creating a new crypto context with the ROC set to zero).
Similarly, if the answerer includes an IP address and/or port that
differs from that used previously for a media stream (or FEC
stream), the answerer MUST include a new master key with the offer
(and hence create a new crypto context with the ROC set to zero).
The reason for this is, that when the answerer receives an offer, or
the offerer receives an answer, with an updated IP address and/or
port, it is not possible to determine if the other side has access
to the old crypto context parameters (and in particular the ROC) or
not. For example, if one side is a decomposed gateway, or a back-
to-back user agent is involved, it is possible that the media
endpoint changed and no longer has access to the old crypto context.
By always requiring a new master key in this case, the
answerer/offerer will know that the ROC is zero for this
offer/answer, and any key lifetime constraints will trivially be
satisfied too. Another consideration here applies to media relays;
if the relay changes the media endpoint on one side transparently to
the other side, the relay cannot operate as a simple packet
reflector but will have to actively engage in SRTP packet processing
and transformation (i.e., decryption and re-encryption, etc.).
Finally note, that if the new offer is rejected, the old crypto
parameters remain in place.
7.1.5 Offer/Answer Example
In this example, the offerer supports two crypto suites (f8 and AES).
The a=crypto line is actually one long line, although it is shown as
Andreasen, Baugher & Wing [Page 24]

INTERNET-DRAFT SDP Security Descriptions June, 2005
two lines in this document due to page formatting. The f8 example
shows two inline parameters; as explained in Section 6.1, there may
be one or more key (i.e. inline) parameters in a crypto attribute.
In this way, multiple keys are offered to support key rotation using
a master key index (MKI).
Offerer sends:
v=0
o=sam 2890844526 2890842807 IN IP4 10.47.16.5
s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=marge@example.com (Marge Simpson)
c=IN IP4 168.2.17.12
t=2873397496 2873404696
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=crypto:2 F8_128_HMAC_SHA1_80
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
FEC_ORDER=FEC_SRTP
Answerer replies:
v=0
o=jill 25690844 8070842634 IN IP4 10.47.16.5
s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=homer@example.com (Homer Simpson)
c=IN IP4 168.2.17.11
t=2873397526 2873405696
m=audio 32640 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
In this case, the session would use the AES_CM_128_HMAC_SHA1_80
crypto suite for the RTP and RTCP traffic. If F8_128_HMAC_SHA1_80
were selected by the answerer, there would be two inline keys
associated with the SRTP cryptographic context. One key has an MKI
value of 1 and the second has an MKI of 2.
7.2 SRTP-Specific Use Outside Offer/Answer
Use of SRTP security descriptions outside the offer/answer model is
not defined.
Use of SRTP security descriptions outside offer/answer could have
been defined for sendonly media streams, however, there would not
Andreasen, Baugher & Wing [Page 25]

INTERNET-DRAFT SDP Security Descriptions June, 2005
be a way to indicate the key to use for SRTCP by the receiver of
said media stream.
7.3 Support for SIP Forking
As mentioned earlier, the security descriptions defined here do not
support multicast media streams or multipoint unicast streams.
However, in the SIP protocol, it is possible to receive several
answers to a single offer due to the use of forking (see [SIP]).
Receiving multiple answers leads to a couple of problems for the
SRTP security descriptions:
* Different answerers may choose different ciphers, keys, etc.,
however there is no way for the offerer to associate a particular
incoming media packet with a particular answer.
* Two or more answerers may pick the same SSRC and hence the SSRC
collision problems mentioned earlier may arise.
As stated earlier, the above point-to-multipoint cases are outside
the scope of the SDP security descriptions. However, there is a way
of supporting SIP forking: Change the multipoint scenario resulting
from SIP forking into multiple two-party unicast cases. This is
done as follows:
For each answer received beyond the initial answer, issue a new
offer to that particular answerer using a new receive transport
address (IP address and port); note that this requires support for
the SIP UPDATE method [RFC 3313]. Also, to ensure that two media
sessions are not inadvertently established prior to the UPDATE being
processed by one of them, use security preconditions [sprecon].
Finally, note that all SIP User Agents that received the offer will
know the key(s) being proposed by the initial offer. If the offerer
wants to ensure security with respect to all other User Agents that
may have received the offer, a new offer/answer exchange with a new
key needs to be performed with the answerer as well. It should be
noted, that the offerer cannot determine whether a single or
multiple SIP User Agents received the offer, since intermediate
forking proxies may only forward a single answer to the offerer.
7.4 SRTP-Specific Backwards Compatibility Considerations
It is possible that the answerer supports the SRTP transport and
accepts the offered media stream, yet it does not support the crypto
attribute defined here. The offerer can recognize this situation by
seeing an accepted SRTP media stream in the answer that does not
include a crypto line. In that case, the security negotiation
defined here MUST be deemed to have failed.
Andreasen, Baugher & Wing [Page 26]

INTERNET-DRAFT SDP Security Descriptions June, 2005
Also, if a media stream with a given SRTP transport (e.g.,
"RTP/SAVP") is sent to a device that does not support SRTP, that
media stream will be rejected.
7.5 Operation with KEYMGT= and k= lines
An offer MAY include both "a=crypto" and "a=keymgt" lines [keymgt].
Per SDP rules, the answerer will ignore attribute lines that it does
not understand. If the answerer supports both "a=crypto" and
"a=keymgt", the answer MUST include either "a=crypto" or "a=keymgt"
but not both, as including both is undefined.
An offer MAY include both "a=crypto" and "k=" lines [SDP]. Per SDP
rules, the answerer will ignore attribute lines it does not
understand. If the answerer supports both "a=crypto" and "k=", the
answer MUST include either "a=crypto" or "k=" but not both, as
including both is undefined.
8 Security Considerations
Like all SDP messages, SDP messages containing security
descriptions, are conveyed in an encapsulating application protocol
(e.g., SIP, MGCP, etc.). It is the responsibility of the
encapsulating protocol to ensure the protection of the SDP security
descriptions. Therefore, IT IS REQUIRED that the application invoke
its own security mechanisms (e.g., secure multiparts such as S/MIME
[smime]) or alternatively utilize a lower-layer security service
(e.g., TLS, or IPsec). IT IS REQUIRED that this security service
provide strong message authentication and packet-payload encryption
as well as effective replay protection.
The discussion which follows uses "message authentication" and
"message confidentiality" in a consistent manner with SRTP
[RFC3711]. "Message confidentiality" means that only the holder of
the secret decryption key can access the plain-text content of the
message. The decryption key is the same key as the encryption key
using SRTP counter mode and f8 encryption transforms, which are
vulnerable to message tampering and need SRTP message authentication
to detect such tampering. "Message authentication" and "message
integrity validation" generally mean the same thing in IETF security
standards: An SRTP message is authenticated following a successful
HMAC integrity check [RFC3711], which proves that the message
originated from the holder of an SRTP master key and was not altered
en route. Such an "authentic" message, however, can be captured by
an attacker and "replayed" when the attacker re-inserts the packet
into the channel. A replayed packet can have a variety of bad
effects on the session, and SRTP uses the extended sequence number
to detect replayed SRTP packets [RFC3711].
Andreasen, Baugher & Wing [Page 27]

INTERNET-DRAFT SDP Security Descriptions June, 2005
The SRTP specification identifies which services and features are
default values that are normative-to-implement (such as
AES_CM_128_80) versus normative-to-use (such as AES_CM_128_32).
8.1 Authentication of packets
Security descriptions as defined herein signal security services for
RTP packets. RTP messages are vulnerable to a variety of attacks
such as replay and forging. To limit these attacks, SRTP message
integrity mechanisms SHOULD be used (SRTP replay protection is
always enabled).
8.2 Keystream Reuse
SRTP security descriptions signal configuration parameters for SRTP
sessions. Misconfigured SRTP sessions are vulnerable to attacks on
their encryption services when running the crypto suites defined in
Sections 6.2.1, 6.2.2, and 6.2.3. An SRTP encryption service is
"misconfigured" when two or more media streams are encrypted using
the same keystream of AES blocks. When senders and receivers share
derived session keys, SRTP requires that the SSRCs of session
participants serve to make their corresponding keystreams unique,
which is violated in the case of SSRC collision: SRTP SSRC collision
drastically weakens SRTP or SRTCP payload encryption during the time
that identical keystreams were used [srtp]. An attacker, for
example, might collect SRTP and SRTCP messages and await a
collision. This attack on the AES-CM and AES-f8 encryption is
avoided entirely when each media stream has its own unique master
key in both the send and receive direction. This specification
restricts use of SDP security description to unicast point-to-point
streams so that keys are not shared between SRTP hosts, and the
master keys used in the send and receive direction for a given media
stream are unique.
8.3 Signaling Authentication and Signaling Encryption
There is no reason to incur the complexity and computational expense
of SRTP, however, when its key establishment is exposed to
unauthorized parties. In most cases, the SRTP crypto attribute and
its parameters are vulnerable to denial of service attacks when they
are carried in an unauthenticated SDP message. In some cases, the
integrity or confidentiality of the RTP stream can be compromised.
For example, if an attacker sets UNENCRYPTED for the SRTP stream in
an offer, this could result in the answerer not decrypting the
encrypted SRTP messages. In the worst case, the answerer might
itself send unencrypted SRTP and leave its data exposed to snooping.
Thus, IT IS REQUIRED that MIME secure multiparts, IPsec, TLS, or
some other data security service be used to provide message
authentication for the encapsulating protocol that carries the SDP
messages having a crypto attribute (a=crypto). Furthermore, IT IS
Andreasen, Baugher & Wing [Page 28]

INTERNET-DRAFT SDP Security Descriptions June, 2005
REQUIRED that encryption of the encapsulating payload be used
whenever a master key parameter (inline) appears in the message.
Failure to encrypt the SDP message containing an inline SRTP master
key renders the SRTP authentication or encryption service useless in
practically all circumstances. Failure to authenticate an SDP
message that carries SRTP parameters renders the SRTP authentication
or encryption service useless in most practical applications.
When the communication path of the SDP message is routed through
intermediate systems that inspect parts of the SDP message, security
protocols such as IPsec or TLS SHOULD NOT be used for encrypting
and/or authenticating the security description. In the case of
intermediate-system processing of a message containing SDP security
descriptions, the "a=crypto" attributes SHOULD be protected end-to-
end so that the intermediate system can neither modify the security
description nor access the keying material. Network or transport
security protocols that terminate at each intermediate system,
therefore, SHOULD NOT be used for protecting SDP security
descriptions. A security protocol SHOULD allow the security
descriptions to be encrypted and authenticated end-to-end
independently of the portions of the SDP message that any
intermediate system modifies or inspects: MIME secure multiparts
are RECOMMENDED for the protection of SDP messages that are
processed by intermediate systems.
9 Grammar
In this section we first provide the ABNF grammar for the generic
crypto attribute, and then we provide the ABNF grammar for the SRTP
specific use of the crypto attribute.
9.1 Generic "Crypto" Attribute Grammar
The ABNF grammar for the crypto attribute is defined below:
"a=crypto:" tag 1*WSP crypto-suite 1*WSP key-params
*(1*WSP session-param)
tag = 1*9DIGIT
crypto-suite = 1*(ALPHA / DIGIT / "_")
key-params = key-param *(";" key-param)
key-param = key-method ":" key-info
key-method = "inline" / key-method-ext
key-method-ext = 1*(ALPHA / DIGIT / "_")
key-info = %x21-3A / %x3C-7E ; visible (printing) characters
; except semi-colon
session-param = 1*(VCHAR) ; visible (printing) characters
where WSP, ALPHA, DIGIT, and VCHAR are defined in [RFC2234].
Andreasen, Baugher & Wing [Page 29]

INTERNET-DRAFT SDP Security Descriptions June, 200510 IANA Considerations10.1 Registration of the "crypto" attribute
The IANA is hereby requested to register a new SDP attribute as
follows:
Attribute name: crypto
Long form name: Security description cryptographic attribute
for media streams
Type of attribute: Media-level
Subject to charset: No
Purpose: Security descriptions
Appropriate values: See Section 410.2 New IANA Registries and Registration Procedures
The following sub-sections define a new IANA registry with associated
sub-registries to be used for the SDP security descriptions. The
IANA is hereby requested to create an SDP Security Description
registry as shown below and further described in the following
sections:
SDP Security Descriptions
|
+- Key Methods (described in 10.2.1)
|
+- Media Stream Transports (described in 10.2.2)
|
+- Transport1 (e.g. SRTP)
| |
| +- Supported Key Methods (e.g. inline)
| |
| +- crypto suites
| |
| +- session parameters
|
+- Transport2
: :
10.2.1 Key Method Registry and Registration
The IANA is hereby requested to create a new subregistry for SDP
security description key methods. An IANA key method registration
MUST be documented in an RFC in accordance with the RFC 2434
Standards Action and it MUST provide the name of the key method in
accordance with the grammar for key-method-ext defined in Section9.1.
Andreasen, Baugher & Wing [Page 31]

INTERNET-DRAFT SDP Security Descriptions June, 200510.2.2 Media Stream Transport Registry and Registration
The IANA is hereby requested to create a new subregistry for SDP
security description Media Stream Transports. An IANA media stream
transport registration MUST be documented in an RFC in accordance
with the RFC 2434 Standards Action and the procedures defined in
Section 4 and 5 of this document. The registration MUST provide the
name of the transport and a list of supported key methods.
In addition, each new media stream transport registry must contain a
crypto-suite registry and a session parameter registry as well as
IANA instructions for how to populate these registries.
10.3 Initial Registrations10.3.1 Key Method
The following security descriptions key methods are hereby
registered:
inline
10.3.2 SRTP Media Stream Transport
The IANA is hereby requested to create an SDP Security Description
Media Stream Transport subregistry for "SRTP". The key methods
supported is "inline". The reference for the SDP security
description for SRTP is this document.
10.3.2.1 SRTP Crypto Suite Registry and Registration
The IANA is hereby requested to create a new subregistry for SRTP
crypto suites under the SRTP transport of the SDP Security
Descriptions. An IANA SRTP crypto suite registration MUST indicate
the crypto suite name in accordance with the grammar for srtp-
crypto-suite-ext defined in Section 9.2.
The semantics of the SRTP crypto suite MUST be described in an RFC
in accordance with the RFC 2434 Standards Action, including the
semantics of the "inline" key-method and any special semantics of
parameters.
The following SRTP crypto suites are hereby registered:
AES_CM_128_HMAC_SHA1_80
AES_CM_128_HMAC_SHA1_32
F8_128_HMAC_SHA1_80
The reference for these crypto-suites is provided in this document.
Andreasen, Baugher & Wing [Page 32]

INTERNET-DRAFT SDP Security Descriptions June, 200510.3.2.2 SRTP Session Parameter Registration
The IANA is hereby requested to create a new subregistry for SRTP
session parameters under the SRTP transport of the SDP Security
Descriptions. An IANA SRTP session parameter registration MUST
indicate the session parameter name (srtp-session-extension as
defined in Section 9.2); the name MUST NOT begin with the dash
character ("-").
The semantics of the parameter MUST be described in an RFC in
accordance with the RFC 2434 Standards Action. If values can be
assigned to the parameter, then the format and possible values that
can be assigned MUST be described in the RFC in accordance with the
RFC 2434 Standards Action as well. Also, it MUST be specified
whether the parameter is declarative or negotiated in the
offer/answer model.
The following SRTP session parameters are hereby registered:
KDR
UNENCRYPTED_SRTP
UNENCRYPTED_SRTCP
UNAUTHENTICATED_SRTP
FEC_ORDER
FEC_KEY
WSH
The reference for these parameters is this document.
11 Acknowledgements
This document is a product of the IETF MMUSIC working group and has
benefited from comments from its participants. This document also
benefited from discussions with Elisabetta Cararra, Earl Carter,
Bill Foster, Matt Hammer, Cullen Jennings, Paul Kyzivat, David
McGrew, Mats Naslund, Dave Oran, Jonathan Rosenberg, Dave Singer,
Mike Thomas, Brian Weis, and Magnus Westerlund.
12 Authors' Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, New Jersey 08837 USA
fandreas@cisco.com
Mark Baugher
5510 SW Orchid Street
Portland, Oregon 97219 USA
mbaugher@cisco.com
Andreasen, Baugher & Wing [Page 33]

INTERNET-DRAFT SDP Security Descriptions June, 2005
[skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange
Mechanism for the Internet", ISOC Secure Networks and Distributed
Systems Symposium, San Diego, 1996.
[RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC3312, October 2002, http://www.ietf.org/rfc/rfc3312.txt.
[RFC2974] M. Handley, C. Perkins, E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000,
http://www.ietf.org/rfc/rfc2974.txt.
[srtpf] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)", work in progress, October 2003.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
15 Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to
rights in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
Andreasen, Baugher & Wing [Page 36]

INTERNET-DRAFT SDP Security Descriptions June, 2005
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to the
IETF at ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Andreasen, Baugher & Wing [Page 37]